This invention relates to improvements in aluminum alloy for use in armoured cable wrap which permits the use of less aluminum alloy in the cable wrap, achieving lighter construction whilst still meeting cable strength requirements. It also relates to an alloy strip for use in making the cable wrap and to the armoured cable wrap formed from the strip.
Metal armoured electrical cables have been known for many years in which an electrical conduit is contained within a metal wrap or sheath. This armour wrap is typically formed from steel or aluminum alloys with a thin strip of metal being formed into a spiral with an overlap between each turn or convolution of the strip. When formed into a wrap, the metal strip typically takes on an "S" curve shape in cross-section with varying wall thickness. The metal strips used for this purpose are supplied in a number of different sizes depending upon the diameter of the cable. Typical thicknesses and widths respectively are (a) 0.025 inches.times.0.375 inches (b) 0.034 inches.times.0.5 inches (c) 0.04 inches.times.0.75 inches and (d) 0.06 inches.times.1.0 inch. The flexibility of the cable or a particular design is typically governed by the number of turns per unit length of armour wrap, and Underwriters Laboratories, for example, specify a minimum of 43 turns per foot of length for the 0.025 inches.times.0.375 inches (0.64 mm.times. 9.5 mm) strip.
In addition to meeting flexibility requirements, armoured cable wrap must meet various strength standards, for example crush resistance, but more particularly with regard to tension or pull-out. Of course, there is at the same time always the desire to achieve as light a construction as possible.
In attempting to produce lightweight cable through use of aluminum alloys, cable manufacturers have attempted to reduce the amount of material required in the cable wrap by using designs with as few turns per unit length as possible and as thin a wall section as possible. However, they have been limited by the requirements of strength standards. Typically, in order to meet strength standards, manufacturers are required to produce armour electrical cable (for wrap produced from 0.025.times.0.375 inch strip) meeting Underwriters Laboratories minimum pull test requirements of 300 pounds (at 100% pass rate). For aluminum alloy armour wrap, this requires as many as 49 to 50 turns/foot and a finished wall thickness of 0.02 to 0.025 inches (0.5 to 0.6 mm) to pass. As an alternative, steel can be used for producing the armour wrap, which allows the strength requirements to be met, but with a penalty on weight.
In order to produce aluminum alloy cable wrap with minimum material requirements, it is necessary to be able to reduce the number of turns per foot of armour wrap down close to the minimum of 43 turns/foot permitted by regulations for 0.025.times.0.375 inch strip and to be able to use a minimum thickness wall construction. However, current aluminum alloy materials do not permit this. In order to form an armour wrap, a high degree of formability is needed to make the narrow radius bends that are required and such formability is not found in available aluminum alloys accompanied by the high strength needed to assure that the strength tests can be met.
Typical alloys that have been used for this purpose include Aluminum Association designated alloys: AA3004 (nom. 1.2% Mn, 1.0% Mg), AA5052 (nom. 2.5% Mg, 0.25% Cr), AA5154 (nom. 3.5% Mg, 0.25% Cr). Other alloys have been proposed. For example Yanagida et al, U.S. Pat. No. 3,961,944, issued Jun. 8, 1976 discloses an alloy containing less than 1.7% Mn and less than 0.8% Cr with optional Li intended for use in armour wrap with low eddy current characteristics while retaining high formability.
It is, therefore, the purpose of the present invention to provide an aluminum alloy material which can be used to produce lightweight armour cable wrap which meets or exceeds the requirements of Underwriters Laboratories (UL) tests for strength.